It is well known to provide components of machinery, such as certain critical components in gas turbine engines, with cooling systems so that the machinery may be operated at higher temperatures than would be possible without such cooling systems. The higher operating temperatures permitted by such cooling systems result in increased performance and efficiency.
To design an optimum cooling system for components which will operate in a high temperature environment, it is necessary to determine the heat transfer coefficient distribution on predetermined surfaces of the cooled components. In the past, determination of the heat transfer coefficient distribution on the surface of a component was performed by extrapolating test measurements obtained from a scale model of the component.
There are, however, problems with such an approach caused by two requirements, one, that the model be affordable and, two, that it be large enough to permit the installation of instrumentation required to measure parameters which permit a determination of the heat transfer coefficient distribution. These requirements determined the degree of geometrical similarity between the model and the actual hardware and thus the closeness of the test measurement conditions to actual conditions.
Cost considerations often dictated that the geometry of the scale model was much simpler than that of the actual component. For example, gas turbine engine nozzles were usually modeled in two dimensional cascades because the cost of building and instrumenting a three dimensional cascade was prohibitive. As a result, the geometry of the model differed from that of the actual hardware.
In the case of turbine engine nozzle cascades, the geometry of the two dimensional model could not only differ, but could also differ significantly, from the actual engine geometry. In the engine geometry, the vanes in the cascade may have a twist or a variable cross section, or both. There is usually no twist or change in cross section in a model. Also in the engine geometry, the axial profile of the endwall might be contoured, whereas, in the model, the axial profile of the endwall was not contoured. The radius of the endwall in an engine is finite, whereas the endwall of the model was flat and thus had an infinite radius. The effect of these differences between the engine and the model geometries had to be accounted for by analysis if an accurate indication of engine characteristics was to be obtained from measurements on the model. The need for this analysis partially compromised the validity and accuracy of any test results.
In addition to simplifications resulting from the need to make the model affordable, it was often necessary to make a model which was much larger than the actual component so that the necessary measuring instruments could be installed. Models were frequently built on a scale of ten to one in each of three dimensions as compared with actual hardware, which resulted in test models which were 1000 times the volume of the actual hardware. To account for this large difference in size between the model and the hardware, the test results generally were reduced to dimensionless form. An assumption then was made that the dimensionless form accounted for the large differences in size. However, this assumption was not always valid and was seldom if ever verified.
In any event, the differences in geometry and size between the scale model and the actual component compromised the validity of heat transfer coefficient distribution test measurements obtained from the scale model. As a result, cooling systems designed with measurements obtained from a scale model often did not provide optimum cooling.
Consequently, a long felt but unfulfilled need has existed for a test which will produce accurate measurements of the heat transfer coefficient distribution on a predetermined surfaces of cooled components of machinery such as gas turbine engines.